Effects of Solvent Composition and Hydrogen Pressure on the Catalytic Conversion of 1,2,4,5-Tetrachlorobenzene to Cyclohexane

Toward the development of a “green” technology for cleaning soil contaminated by halogenated hydrophobic organic contaminants, here we demonstrate that combined use of palladium (Pd) and rhodium (Rh) catalysts enables the conversion of 1,2,4,5-tetrachlorobenzene (TeCB) to cyclohexane in mixtures of water and ethanol. We tested the hypotheses that, in batch reactors, (1) an increased ratio of water to ethanol in water/ethanol solvents would increase the reaction rates of both Pd-catalyzed hydrodehalogenation (HDH) and Rh-catalyzed hydrogenation, and (2) catalytic reaction rate coefficients would be constant above a hydrogen (H2) pressure threshold, but would decrease with decreasing H2 pressure below that threshold. These hypotheses were derived from a Langmuir–Hinshelwood model for the heterogeneous catalytic reactions. Complete conversion of TeCB to cyclohexane was achieved at all experimental conditions tested, suggesting that the proposed technology may be technically viable. Concentration data were consistent with an apparent first-order kinetic model in which Pd-catalyzed HDH and Rh-catalyzed hydrogenation occur in series. As expected, HDH and hydrogenation rate coefficients increased as the fraction of water in the solvent increased. However, contrary to expectations, HDH rate coefficients decreased when H2 pressure increased from 69 to 207 to 345 kPa. We attributed this to the displacement of TeCB by H2 on the catalyst surface at higher H2 pressures. No statistically significant effect of H2 pressure on hydrogenation rate coefficients was observed. The findings suggest that the proposed technology should be operated with at least 50% water in the solvent and a H2 pressure as low as 30–70 kPa

Biodegradation of polyfluorinated biphenyl in bacteria

Fluorinated aromatic compounds are significant environmental pollutants, and microorganisms play important roles in their biodegradation. The effect of fluorine substitution on the transformation of fluorobiphenyl in two bacteria was investigated. Pseudomonas pseudoalcaligenes KF707 and Burkholderia xenovorans LB400 used 2,3,4,5,6-pentafluorobiphenyl and 4,4′-difluorobiphenyl as sole sources of carbon and energy. The catabolism of the fluorinated compounds was examined by gas chromatography–mass spectrometry and fluorine-19 nuclear magnetic resonance spectroscopy (19F NMR), and revealed that the bacteria employed the upper pathway of biphenyl catabolism to degrade these xenobiotics. The novel fluorometabolites 3-pentafluorophenyl-cyclohexa-3,5-diene-1,2-diol and 3-pentafluorophenyl-benzene-1,2-diol were detected in the supernatants of biphenyl-grown resting cells incubated with 2,3,4,5,6-pentafluorobiphenyl, most likely as a consequence of the actions of BphA and BphB. 4-Fluorobenzoate was detected in cultures incubated with 4,4′-difluorobiphenyl and 19F NMR analysis of the supernatant from P. pseudoalcaligenes KF707 revealed the presence of additional water-soluble fluorometabolites.Author has checked copyrightAM

Integration of Matrix-Assisted Laser Desorption Ionization–Time of Flight Mass Spectrometry and Molecular Cloning for the Identification and Functional Characterization of Mobile ortho-Halobenzoate Oxygenase Genes in Pseudomonas aeruginosa Strain JB2

Protein mass spectrometry and molecular cloning techniques were used to identify and characterize mobile o-halobenzoate oxygenase genes in Pseudomonas aeruginosa strain JB2 and Pseudomonas huttiensis strain D1. Proteins induced in strains JB2 and D1 by growth on 2-chlorobenzoate (2-CBa) were extracted from sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels and analyzed by matrix-assisted laser desorption ionization–time of flight mass spectrometry. Two bands gave significant matches to OhbB and OhbA, which have been reported to be the α and β subunits, respectively, of an ortho-1,2-halobenzoate dioxygenase of P. aeruginosa strain 142 (T. V. Tsoi, E. G. Plotnikova, J. R. Cole, W. F. Guerin, M. Bagdasarian, and J. M. Tiedje, Appl. Environ. Microbiol. 65:2151–2162, 1999). PCR and Southern hybridization experiments confirmed that ohbAB were present in strain JB2 and were transferred from strain JB2 to strain D1. While the sequences of ohbA from strains JB2, D1, and 142 were identical, the sequences of ohbB from strains JB2 and D1 were identical to each other but differed slightly from that of strain 142. PCR analyses and Southern hybridization analyses indicated that ohbAB were conserved in strains JB2 and D1 and in strain 142 but that the regions adjoining these genes were divergent. Expression of ohbAB in Escherichia coli resulted in conversion of o-chlorobenzoates to the corresponding (chloro)catechols with the following apparent affinity: 2-CBa ≈ 2,5-dichlorobenzoate > 2,3,5-trichlorobenzoate > 2,4-dichlorobenzoate. The activity of OhbAB(JB2) appeared to differ from that reported for OhbAB(142) primarily in that a chlorine in the para position posed a greater impediment to catalysis with the former. Hybridization analysis of spontaneous 2-CBa(−) mutants of strains JB2 and D1 verified that ohbAB were lost along with the genes, suggesting that all of the genes may be contained in the same mobile element. Strains JB2 and 142 originated from California and Russia, respectively. Thus, ohbAB and/or the mobile element on which they are carried may have a global distribution